Photographic enlarger with electronic light control

ABSTRACT

An electronic light control system for photographic enlargers. A relay controlled by an electronic circuit turns the main enlarger light on to give the necessary illumination during the preliminary operations, then turns it off, and turns it on again for a controlled time interval for making the exposure on the sensitized paper. The control switches, which may be at a control desk or console, are in electronic circuits which may operate at a lower voltage than the main enlarger lamp, so that these control switches and the conductors leading to them do not have to withstand the higher voltage of the enlarger lamp.

United States Patent 1191 Stiller 1 1 July 15, 1975 PHOTOGRAPHIC ENLARGER WIT-H 3.211.029 g; lYcaln 13177/1 S L ,9.81 11 j"O11SO1l ..0115X ELECTRONIC LIGHT CONTRO I 3.737734 Q6/1973: u [75] Inventor: Armin Stiller, Braunschweig, 3,742,295 1973 Germany 1 3,754,165 8/1973 73 Assignee: Rollei-Werke Franke & Heidecke, 4/1974 Kay 7/1485 8 x Braunschwelg Germany Primary Examiner.lames W. Lawrence [22] Filed: Feb. 11, 1974 Assistant Examiner-E. R. LaRoche Attorney, Agent, or FirmStonebraker, Shepard & [21] Appl. No.. 441,484 Stephens [30] Foreign Application Priority Data [5 ABSTRACT Feb. 14, 1973 Germany 2307200 n electronic light control system for photographic enlargers. A relay controlled by an electronic circuit [52] US Cl- 315/360; 307/ 141.8; 315/208; turns the main enlarger light on to give the necessary 317/141 S; 317/ 148.5 B; 355/69 illumination during the preliminary operations, then [51] Int. Cl G03b 27/76; H0111 47/32 turns it off, and turns it on again for a controlled time [58] Field 01 Search 315/119, 200 R, 208, 360; interval for making the exposure on the sensitized pa- 307/115, 141, 141.4, 141.8, 157, 293; per. The control switches, which may be at a control 317/135 A, 141 S, 148.5 B; 355/69 desk or console, are in electronic circuits which may I operate at a lower voltage than the main enlarger [56] References Cit d lamp, so that these control switches and the conduc- UNITED STATES PATENTS tors leading to them do not have to withstand the 3,555,390 1/1971 Peffer et al 317/1485 B x "wage of the enlarger lamp' 3,571,612 3/1971 Ukai et a1. 307/1418 2 Claims, 1 Drawing Figure PHOTOGRAPHIC ENLARGER WITH ELECTRONIC LIGHT CONTROL BACKGROUND OF THE INVENTION In photographic enlargers, it is necessary to turn the enlarger lamp on while doing some preliminary work in making the enlarger ready for the actual exposure. Such preliminary work may include focusing the image of the negative onto the receiving surface, adjusting the position of the negative carrier and the receiving surface, cropping the image if desired, etc. Then when the preliminary work is completed, the enlarge lamp is turned off, the sensitized paper is placed in position, and the lamp is turned on again for the required controlled interval, to make the actual exposure. Such procedures are well known in the art.

It has been suggested in the prior art to control the switching of the enlarger lamp by a circuit including a thyristor, but in the prior art such circuits have proven to be unreliable in operation.

It is an object of the present invention to provide a more reliable electronic circuit for controlling the illumination lamp of a photographic enlarger.

Another object is the provision of an arrangement whereby the control switches, placed at a desk or console, are in circuits operating at lower voltage than the main line voltage of the illumination lamp, so that the switches themselves and the wiring circuits relating thereto do not have to withstand the full voltage of the lamp and therefore can be made of lower quality and less expensive materials.

The illumination lamp of a photographic enlarger is in the lamp head at the top of the enlarger, which is at some distance, at least a few feet, from the operating desk or console where the control switches are conveniently grouped. It is another object of the invention to provide control circuits so designed and arranged that only conductors carrying lower voltages need to run from the operating desk or console to the enlarger head containing the lamp, thus enabling the control to be accomplished in a very simple and inexpensive way, without the necessity of running special cables or leads from the console to the enlarger head, of sufficiently heavy capacity to carry the mains or line voltage at which the illumination lamp operates.

SUMMARY OF THE INVENTION According to the invention, there is a relay switch which controls the supply of current to the illumination lamp, and a thyristor in the circuit of the relay. There is also a control switch, by the closing of which it is possible to apply to the ignition section or control gate of the thyristor, for a short time, a voltage which causes the thyristor to be driven hard.

According to one particularly advantageous feature of the invention, this switch is connected to the cathode of the thyristor bridged with a resistor, while a series circuit consisting of a resistor and a capacitor is arranged in parallel with the switch, and the resistor of the ignition section or control gate of the thyristor is connected in parallel.

This insures that when the device for controlling the illumination lamp of the enlarger is switched on, the thyristor can not immediately ignite as soon as it receives a control impulse from the timing circuit, and that it will only ignite after the switch situated in its current circuit has been closed. This enables the exposure lamp to be kept on as long as desired, to enable as much preparatory or preliminary work as desired to be accomplished, without having the lamp prematurely turned off. After the preparatory work has been completed, the switch in the current circuit of the thyristor is closed, so that the latter cuts through and the illumination lamp is extinguished. Then the exposure process can take place in the conventional manner, with the aid of a timing switch.

BRIEF DESCRIPTION OF THE DRAWING The single view of the drawing is a wiring diagram illustrating a preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the wiring diagram, the terminals 1 and 2 of the switching arrangement or control arrangement of the present invention are connected to any suitable source of direct current, schematically indicated at 30, which may be a conventional converter having an output of direct current, from an input of alternating current received from the conventional supply line or mains 4. The enlarger exposure lamp 3 also receives its current from the mains 4. A main on-andoff switch 4a is provided, which turns off both the lamp 3 and the converter 30 when this switch is open. The lamp 3 is situated, as usual, in the head of the enlarger, and the current to the lamp is controlled by a relay switching contact 5 which is opened when the relay coil or winding 6 is energized, and is normally closed when no current flows through the relay 6. This relay 6 is part of the electronic control circuit which will now be described.

In parallel with the terminals 1 and 2 is a series connection including the exciter winding or coil of the relay 6, a thyristor 7, and a single pole switch 8. The thyristor is bridged by a resistor 9 in parallel with the thyristor as shown. A series circuit having a resistor 10 and capacitor 11 is in parallel with the switch 8. This resistor 10 is also connected in parallel with a compensating resistor 28 in the ignition section or gate circuit of the thyristor. A series connection of a further resistor 12 and a time switch 13 extends from the terminal 1 to the terminal 2, that is, in parallel with the above described circuit having the elements 6, 7, and 8. A quenching capacitor 14 is connected as illustrated, from a point between the resistor 12 and the switch 13, to the anode of the thyristor 7.

A Zener diode 15 is connected in the manner illustrated in the wiring diagram. With the thyristor 7 blocked and the switch 8 closed, the timing circuit is subjected, by means of this Zener diode 15, to a constant direct voltage, inasmuch as the Zener diode is parallel to the series circuit comprising the thyristor 7 and the switch 8, or the resistor 9 and the switch 8. The timing circuit comprises, in known manner, a a programmable unijunction transistor 16, the timing elements 17, 18, 19, 20, 21, 22, and 23, and a voltage divider circuit having resistors 24 and 25, this voltage divider circuit being connected with the terminals 1 and 2 in the manner illustrated. The central tap of the voltage divider circuit is connected to the control grid or gate of the programmable unijunction transistor or PUT 16. The timing circuit further comprises a resistor 26 which is connected with the cathode of the PUT and in parallel with the.QQUlrQl-se EiQmQf the thyristor 7,

which control 's'ect'ion is provided for this-purpose, withlij a series resistor 29. The voltage divider circuit 24, 25 is subjected. by means of another Zener diode 27, to a constant voltage.

These and other circuit elements and connections are shown in the wiring diagram and thus will be readily apparent to those skilled in the art, and need not be further described in detail.

The circuit operates as follows:

In the position of rest, the switches 8 and 13 are open, and the switch is closed. this being the normal position of the switch 5 when no current is flowing through the relay 6. When the main switch 4a is closed, the enlarger lamp 3 is directly connected to the alternating current mains 4, at the normal line voltage (say for example l volts) while the terminals 1 and 2 of the timing or control circuit are subjected, through the converter 30, to a direct current voltage which is preferably considerably less than the mains or line voltage in the line 4. The exposure lamp 3 is lit, and the photographic enlarger is illuminated so that the operator can perform the preliminary or preparatory work.

When the preparatory work is completed and the user is ready to make the exposure, the switch 8 is closed. The capacitor 11, which has previously been charged up, through the resistors 9 and 10, to a voltage corresponding to the voltage drop across the resistors 28, 29, and 26, now discharges through the resistor 10. The ignition section or gate of the thyristor 7 thus receives an ignition voltage, and the thyristor, which is subjected to the voltage across the terminals 1 and 2 when the switch 8 is closed, is driven hard. A current thus flows through the winding of the relay 6, and this opens the switch 5, so that the enlarger illumination lamp 3 is extinguished. The capacitor 14 now charges up, through the resistor 12, to the terminal voltage.

When the operator is ready to make the actual exposure, he closes the switch 13, without touching the switch 8, which remains closed. The capacitor 14 then discharges through the closed switch 13, the closed switch 8, and the cathode/anode circuit of the thyristor 7, so that the commutation current causes the thyristor to turn off or become non-conductive. Expressing it another way, the charge carriers in the PN junction of the thyristor are discharged and the thyristor blocks. The flow of current in the winding of the relay 6 is thus interrupted, so that the relay switch 5 closes, and the enlarger lamp 3 becomes lighted.

Beginning at the moment that the thyristor 7 becomes blocked, the timing circuit is subjected to a constant direct voltage corresponding to the breakdown voltage of the Zener diode 15. The capacitors 22 and 23 charge up with a time constant adjustable by means of the adjustable resistors 18, 20, and 21. When the hump voltage of the PUT 16 is reached, the latter becomes conductive and the capacitors 22 and 23 discharge via the PUT 16, in which process a voltage impulse is generated in the resistor 26, This voltage impulse is fed to the gate of the thyristor 7 and causes the thyristor to cut through or become conductive.

When the thyristor 7 thus becomes conductive, current from the terminals 1 and 2 will now flow through the relay 6, since the switch 8 is still closed at this time. The flow of current through the relay 6 opens the relay switch 5, interrupting the current to the exposure lamp 3, so that the lamp is extinguished. This occurrs, as

above indicated, at a controlled time interval after the lamp was lit, the time interval being determined by the previous adjustment of the elements 18, 20 and 21 in accordance with the wishes of the operator as to the length of exposure to be given. The timing circuit is short circuited at the moment when the thyristor 7 is driven hard.

The operator then opens the switch 13. The switch 8 is not opened until the operator has removed the sensitized paper on which the exposure has just been made, and put it in a safe place such as a developer bath, because the opening of the switch 8 will or might cause the relay switch 5 to close and light the lamp 3. Of course the opening of the switch 13 may be postponed, if desired, until the operator is ready to open the switch 8 also. When both of the switches 13 and 8 have been opened, the apparatus is then ready for the next cycle of operation, closing the switch 8 serving to put out the lamp 3 so that the sensitized paper may be placed in exposure position, and the subsequent closing the switch 13 serving to turn on the lamp 3 to make the exposure, for the exposure time interval determined by the timing portion of the circuit.

What is claimed is:

l. A photographic enlarger comprising a lamp circuit, a lamp in said circuit, a first switch in said lamp circuit in series with said lamp, said first switch being normally closed, means for supplying current of one voltage to said lamp circuit to cause said lamp to be lit while said first switch is closed, and an electronic control circuit for controlling the respective lit and unlit conditions of said lamp by operation of said first switch from a location remote from said first switch, said control circuit comprising a source of direct current at a voltage less than the voltage supplied to said lamp circuit, a relay coil (6) for operating said first switch, said first switch being closed while no current is flowing in said relay coil and being open while current is flowing therein, a thyristor (7) in series with said relay coil, a second switch (8) in series with said relay coil and thyristor, said relay coil, thyristor, and second switch being connected in series across said source of direct current, means responsive to the closing of said second switch for rendering said thyristor conductive so that current will flow in said relay coil to open said first switch and extinguish said lamp to darken the enlarger while sensitized paper is placed in exposure position therein, a third switch (13), means responsive to the closing of said third switch for causing a commutation current to flow through said thyristor to render said thyristor nonconductive so that current ceases to flow through said relay coil and said first switch closes to light said lamp and start an exposure interval, exposure timing means for determining the length of an exposure interval, and means responsive to said exposure timing means for causing an impulse to be delivered to a control gate of said thyristor at the end of an interval determined by said timing means, to render said thyristor again conductive, so that current again flows through said thyristor and relay coil to open said first switch and extinguish said lamp.

2. The invention as defined in claim 1, wherein said second switch (8) and said third switch (13) are in locations remote from said lamp (3) and said first switch 

1. A photographic enlarger comprising a lamp circuit, a lamp in said circuit, a first switch in said lamp circuit in series with said lamp, said first switch being normally closed, means for supplying current of one voltage to said lamp circuit to cause said lamp to be lit while said first switch is closed, and an electronic control circuit for controlling the respective lit and unlit conditions of said lamp by operation of said first switch from a location remote from said first switch, said control circuit comprising a source of direct current at a voltage less than the voltage supplied to said lamp circuit, a relay coil (6) for operating said first switch, said first switch being closed while no current is flowing in said relay coil and being open while current is flowing therein, a thyristor (7) in series with said relay coil, a second switch (8) in series with said relay coil and thyristor, said relay coil, thyristor, and second switch being connected in series across said source of direct current, means responsive to the closing of said second switch for rendering said thyristor conductive so that current will flow in said relay coil to open said first switch and extinguish said lamp to darken the enlarger while sensitized paper is placed in exposure position therein, a third switch (13), means responsive to the closing of said third switch for causing a commutation current to flow through said thyristor to render said thyristor non-conductive so that current ceases to flow through said relay coil and said first switch closes to light said lamp and start an exposure interval, exposure timing means for determining the length of an exposure interval, and means responsive to said exposure timing means for causing an impulse to be delivered to a control gate of said thyristor at the end of an interval determined by said timing means, to render said thyristor again conductive, so that current again flows through said thyristor and relay coil to open said first switch and extinguish said lamp.
 2. The invention as defined in claim 1, wherein said second switch (8) and said third switch (13) are in locations remote from said lamp (3) and said first switch (5). 